The invention relates to improved neodymium ultraphosphates which can be used especially as solid laser materials, but for other purposes as well, and to a method of manufacturing same.
Neodymium-doped crystals, such as yttrium-aluminum-garnets (YAG) or yttrium-aluminum oxide (YAlO) and neodymium-doped glasses constitute good solid laser materials, as it is known: in these materials the energy levels of the trivalent neodymium ion which are required for laser operation are rather good, both in regard to pumping conditions (absorption bands in the visible and near-infrared range) and in regard to emission (so-called four-level operation is possible). The most important emission lines are relatively sharp, the best known line being at a wavelength of 1.06 .mu.m. The laser-active neodymium ions do not display any aging or decomposition phenomena. Furthermore, the above-mentioned crystalline materials have a good thermal conductivity (important for lasers of high sustained output) and the glasses can be produced economically in large pieces (important in lasers of high pulse energies).
Nevertheless, these known laser materials have one disadvantage which has lead to a search for still better materials.
For optical communication, optical transmitters and optical amplifiers are needed which can be miniaturized, i.e., laser materials are needed which have a high optical amplification even when their dimensions are very small. Semiconductor lasers and neodymium lasers, basically, come under consideration for this purpose.
In a fundamental scientific paper (H. G. Danielmeyer, M. Blatte and P. Balmer, Appl. Phys. 2, 269-274, 1973) it has been shown that the Nd:YAG neodymium laser, which has hitherto been the best, is not suitable for miniaturization because the neodymium concentration cannot be made sufficiently great. It has been found that, if the Nd concentration is increased to more than a few percent, the fluorescent lifetime of the ions decreases greatly for fundamental reasons, so that the required pumping energy increases in an undesirable manner.
NdP.sub.5 O.sub.14 (neodymium ultraphosphate) is the first laser material that has been produced, not by doping, but as a genuine chemical compound, and which can produce and amplify laser radiation, even continuously, at room temperature, with a high degree of efficiency. By x-ray studies it has been found that NdP.sub.5 O.sub.14 is monoclinic and has a twinning tendency. This latter characteristic, however, is a decided disadvantage as regards the optical quality of the crystal. Furthermore, the duration of the upper laser state (.sup.4 F.sub.3/2, of 66 microseconds, is relatively short and the line width of the principle laser transition at 1.05 .mu.m, which is 50 A, is relatively great, so that the laser threshold is relatively high.
It is the object of the invention, therefore, to overcome the disadvantages of neodymium ultraphosphate and to create an improved material which will have a longer life, will reduce the line width and increase the laser efficiency, and which will have no twinning tendency or will have it to a reduced degree. Another object of the invention is to eliminate the quality variations which have hitherto occurred in neodymium ultraphosphate from different production batches. In the hitherto known process of the manufacture of NdUp crystals, the shape and size has been quite different in repeated growth experiments, although the conditions were kept constant insofar as possible. Also, variations in regard to fluorescent lifetime are to be eliminated and the size of the grown crystals is to be increased.
This purpose is achieved in accordance with the invention by a process for preparing compounds of the general formula EQU Me.sub.x Nd.sub.1-x P.sub.5 O.sub.14
in which Me represents scandium, gallium, yttrium, indium, lanthanum, cerium, gadolinium, lutetium, thallium and/or uranium, and x is a number between 0 and 0.999, in the form of untwinned crystals, which is characterized by the fact that Nd.sub.2 O.sub.3 and Me.sub.2 O.sub.3 are heated in a vessel consisting of fine gold or an inert, dense, carbon-base material with an excess of high-purity, anhydrous phosphoric acid or diphosphoric or polyphosphoric acid at a temperature between about 500.degree. and 600.degree. C., until the desired crystal size is achieved, and then the excess phosphoric acid or polyphosphoric acid is separated.8
The starting substances Nd.sub.2 O.sub.3 and in some cases Me.sub.2 O.sub.3 must be used in a high purity, preferably of 99.999% and more. This puirty is necessary because even traces of other elements, particularly praseodymium, samarium and dysprosium, suppress the neodymium radiation. The specified vessel material is necessary because hot polyphosphoric acid dissolves everything except gold and certain carbonbase materials. Platinum8vessels are especially unsuitable because platinum forms a compound with pyrophosphoric acid.
Glass-like carbons, boron carbide, silicon carbide and diamond have proven especially suitable as vessel materials. They enable the fluorescent lifetime to be increased by a factor of 1.5 over gold vessels, because gold dissolves in small amounts in the phosphoric acid.
The phosphoric acid must, as previously mentioned, be used in an excess above the stoichiometrically necessary amount. Preferably, a ratio of all of the metal oxides to phosphoric acid between 1 : 30 and 1 : 50, by weight, is used. Larger amounts of phosphoric acid produce no advantage, and when amounts less than the stated range are used, the crystal size obtainable is smaller and the crystal quality poorer.
The phosphoric acid can be either anhydrous or it may still have a moisture content within the scope of the invention. If phosphoric acid containing water is used, it is first heated to a relatively moderately elevated temperature until no more free water is present. Typical conditions for this purpose are 10 hours of heating at about 180.degree. to 220.degree. C. The end of the moisture removal phase can be recognized by the fact that the oxides added begin to dissolve. During removal of the moisture it is desirable to8keep the acid under8a flowing shield of inert gas to carry away the water that is released. Suitable inert gases are those which are inert to the substances used and to water, such as for example nitrogen, oxygen and noble gases. When the moisture removal is complete the heating in the sealed chamber is continued. The dehydrating phase can be shortened or entirely eliminated if anhydrous phosphoric acid or diphosphoric acid (H.sub.4 P.sub.2 O.sub.7) or pyrophosphoric acid is used. The oxides of the above-mentioned rare earths also go completely into solution in diphosphoric acid.
Heavy phosphoric acid (D.sub.3 PO.sub.4) or heavy diphosphoric or polyphosphoric acid may also serve as the phosphoric acid in the invention, and the deuterium may be wholly or partially replaced by tritium. When x in the above formula represents, 0, a deuteriated or tritiated phosphoric acid of this sort must be used to enable the desired characteristics of the crystals to be achieved.
Surprisingly it has been found that, through this embodiment of the process of the invention the fluorescent life of the neodymium ultraphosphate crystals can be substantially increased. This results in a correspondingly lower pumping threshold for use as laser material, or in an improved efficiency. The reason for this is not known, but it is assumed that, when tne neodymium ultraphosphate is prepared in normal phosphoric acid, lattice flaws are formed to a very small extent, in which neodymium is replaced by three hydrogen ions. Since hydrogen ions to a great degree quench the excitation energy of neodymium ions, the maximum possible fluorescent life of neodymium ultraphosphate crystals cannot be achieved when they are grown in normal phosphoric acid or in diphosphoric or polyphosphoric acid. It is assumed that, by this preferred procedure, such flaws are occupied by the far heavier deuterium or tritium which, on account of its doubled or tripled mass, cause no quenching or substantially less quenching of the excitation energy. Another special effect which occurs in the use of tritiated phosphoric acid is that the tritium, with a half life of 12 years, is converted to helium and the crystals become entirely free of easy quenching centers. As a result, noedymium ultraphosphate crystals pulled in phosphoric acid containing tritium improve their lasing characteristics in the course of time.
The surprising effect occurs even when, instead of the normal neodymium ultraphosphate crystals of the formula NdP.sub.5 O.sub.14, the doped neodymium ultraphosphates of the general formula EQU Me.sub.x Nd.sub.1-x P.sub.5 O.sub.14
are produced, wherein Me represents scandium, gallium, yttrium, indium, lanthanum, cerium, gadolinium, lutetium, thallium and/or uranium, and x represents a number between 0.001 and 0.999. In this case only a portion of the Nd.sub.2 O.sub.3 used as starting material is replaced by Me.sub.2 O.sub.3.
Phosphoric acid containing heavy water or already freed thereof can be used as heavy or tritiated phosphoric acid (hereinafter called simply phosphoric acid) within the scope of the process of the invention. The statements made above concerning ordinary phosphoric acid apply here accordingly.
The actual crystal growing takes place between about 500 and about 600.degree. C., preferably between 540.degree. and 560.degree. C. At growing temperatures above 600.degree. C. and below 500.degree. C., the crystal quality diminishes importantly. During crystallization, low-polymeric components of polyphosphoric acid escape, as well as water, which here again is to be understood to include heavy water and tritium oxide, and they condense in the colder part of the closed crystallization system. The growth rate of the crystals can be controlled by regulating the temperature of the condensate. This makes it possible in a simple manner to adjust the water vapor partial pressure in the system, which determines the degree of polymerization of the phosphoric acid. In accordance with a special embodiment of the process of the invention, therefore, the polymerization can be controlled by regulating the water vapor partial pressure. As the water vapor partial pressure increases the crystals redissolve; as it diminishes they resume growing. Particularly good results are obtained when the condensate is kept at room temperature.
By recycling the low-polymeric phosphoric acid condensed in the colder zone, the process of the invention can be made into a continuous recrystallization process in which particularly large crystals are obtained.
The growth rate of the crystals can be controlled not only by the above-mentioned preferred embodiment using control of the temperature of the condensate, but also by producing a temperature gradient in the solution, for example by cooling one side of the gold vessel.
After the crystallization is completed, which in the above-described, preferred embodiment takes about 4 to 8 days, residual polyphosphoric acid can be poured while hot through a gold sieve. Adhering phosphoric acid traces are then removed from the crystals by vacuum evaporation or by passing over them an inert gas saturated with water vapor at room temperature.
The cleaning by means of moist inert gas takes place relatively rapidly at about 500.degree. to 600.degree. C. It is therefore possible to completely eliminate the pouring off of the remaining phosphoric acid and to remove all of the excess phosphoric or polyphosphoric acid completely at the growth temperature. For example, the phosphoric acid can be removed entirely at 550.degree. C. in the growth vessel within one day by passing through it argon which has been saturated at room temperature with water vapor.
The water vapor reduces the degree of polymerization and thus the boiling point of the polyphosphoric acid, which therefore evaporates faster and is carried off by the carrier gas. Any inert gas may be used as the carrier gas. Then a very slow and gentle cooling of the crystals can be performed, for example from 550.degree. C. to room temperature over a period of 5 hours. It is also possible to wash the phosphoric acid out with distilled water, but in this case there is the danger that the crystal faces may be dissolved away or the crystals may crack or craze due to thermal shock.
Additional subject matter of the invention is compounds of the general formula EQU Me.sub.x Nd.sub.1-x P.sub.5 O.sub.14
in which Me represents scandium, gallium, yttrium, indium, lanthanum, cerium, gadolinium, lutetium, thallium and/or uranium, and x represents a number between about 0.001 and 0.999. The compounds of the invention are in monoclinic crystals or in amorphous, glassy form. The compounds of the invention can contain one of the above-named metals or a mixture of same. They have the characteristic of not quenching the neodymium fluorescence. Values between 0.02 and 0.95 are preferred for x, but a substantial improvement of the laser characteristics is achieved even when the additives are incorporated in a non-detectable amount, that is, when their presence can be detected only in the growth solution.
Furthermore, neodymium ultraphosphates which have been obtained with heavy or tritiated phosphoric or diphosphoric or polyphosphoric acid by the method described above are additional subject matter of the invention.
The compounds of the invention can be obtained not only in crystallized form but also in glassy form. The transformation can be brought about by heating crystals in a sealed crucible of appropriate material to a temperature of 900.degree. to 1500.degree. C., at an external shielding gas pressure between 1 and 100 atmospheres. By slow cooling a material is obtained which has solidified mostly in glassy form. It is desirable to subject the vessel to an external pressure corresponding approximately to the rising internal pressure, since the noble metal crucibles are not very strong mechanically. In the glassy state the life time of the upper laser state of the compounds of the invention is considerably shorter, but on the other hand pieces of any desired size can be produced in this manner, which is not possible with crystals. The life time of the upper laser state in the glass amounts to approximately 20 microseconds. On account of their high optical quality and the possibility of giving them any desired shape, such glasses are particularly well suited as laser substances and are greatly desirable for many purposes in spite of their reduced life time.